1. Field of the Invention
The present invention relates to a telecentric lens system and a vision measuring instrument.
2. Description of Related Art
Traditionally, a so-called double-telecentric lens system has been known in which a front lens group and a rear lens group are arranged so that a rear focal point of the front lens group coincides with a front focal point of the rear lens group, and a diaphragm is provided at a position of the coinciding focal point.
Such a telecentric lens system has a wide visual field, a large focal depth, and a magnification of image formation determined by a focal length of the front lens group and the rear lens group irrespective of a position of an object. Accordingly, the telecentric lens system is often used to observe and measure an object having a stepped portion such as edge tools, mechanical parts, or assembled electronic parts.
However, in principle, resolution and focal depth are inversely proportional to each other in the telecentric lens system. In other words, when the focal depth is increased, the resolution is reduced. In order to observe an object at a higher resolution after setting a large focal depth, it is required to change a lens to another lens having a higher magnification or separately observe the object with a microscope having a high resolution. Replacing a lens or separately using a microscope is effort-consuming, which makes an observation operation complicated. Thus, operating efficiency is deteriorated. In addition, separately preparing a lens or a microscope is cost-consuming.
Thus, the inventors have suggested a telecentric lens system and a vision measuring instrument to solve such a problem (see Document 1: JP-A-2003-232999).
The telecentric lens system as disclosed in Document 1 includes: a front lens group; a rear lens group which is disposed so that a front focal point of the rear lens group coincides with a rear focal point of the front lens group; and a diaphragm mechanism which is disposed at a position where the front focal point of the rear lens group coincides with the rear focal point of the front lens group. The diaphragm mechanism has a numerical aperture changer for changing a numerical aperture.
Since the diaphragm mechanism is provided with the numerical aperture changer in such a telecentric lens system, the numerical aperture can be changed. In the telecentric lens system, focal depth and resolution are inversely proportional to each other. When the numerical aperture is reduced by the numerical aperture changer, the focal depth can be made large even though the resolution is low. In other words, an object having a stepped portion can be clearly observed to be measured. On the other hand, when the numerical aperture is increased by the numerical aperture changer, the resolution can be made high even though the focal depth is small. In other words, an object can be precisely observed to be measured at a high resolution.
However, since a single magnification is employed in the above-described telecentric lens system, the range of objects to be measured may be limited.
Further, in a telecentric lens system in which zoom lenses are used, it becomes difficult to associate a magnification with a diaphragm diameter of a diaphragm mechanism.